1. Technical Field
The present invention relates generally to a traction control system for a four-wheel drive vehicle in which a torque split system is incorporated.
2. Background Art
Japanese Patent First Publication (tokkai) No. 63-141831 discloses a torque split control system for a four-wheel drive vehicle. This system may be incorporated in a four-wheel drive vehicle which gives priority to rear-wheel driving and operates such that driving torque delivered to the front wheels is increased according to increase in a rotational speed difference between the front and rear wheels to distribute driving torque to all four wheels, an increase ratio of the driving torque to the front wheels is lowered according to rear driven-wheel slippage resulting from increase in lateral acceleration acting on the vehicle body during turns.
Japanese Patent First Publications (tokkai) Nos. 61-268529, 61-285130, and 61-285131 disclose traction control systems for automotive vehicles.
Publication No. 61-268529 discloses a system wherein a minimum value is selected from among the rotational speeds of all wheels and an estimated vehicle speed to define this value as a threshold for reducing driving torque to preventing wheel spin from occurring.
Publication No. 61-285130 discloses a system which is adapted for reducing driving torque to prevent wheel spin from occurring based on an average value of the rotational speeds of all wheels.
Publication No. 61-285131 discloses a system which is adapted for selecting a minimum speed from among rotational speeds of all wheels to provide a threshold for reducing a driving torque to prevent wheel spin from occurring.
Further, Japanese Patent Publication No. 63-170129 discloses a system in which both a torque split controller and a traction controller are incorporated. This system is operable to engage a clutch disposed in a drive system before effecting traction control to establish four-wheel driving.
If a four-wheel drive vehicle is provided which has a simple combination of a torque split controller and a traction controller, the traction controller monitors wheel spin based on a rotational speed of any one wheel or an average speed of all four wheels to effect traction control. Thus, if yaw motion of the vehicle is controlled by the torque split control to allow a difference in rotational speeds between the front and rear wheels to be created according to turning conditions, the traction control must disenable the torque split control of yaw motion within a certain range to allow operation of the traction control. This results in control interference between both the traction control and torque split control.
In a case of a vehicle including only a torque split control system, a yaw moment, or an attitude angle of a vehicle body can be controlled, however this causes excess driving torque of the four wheels. Thus, reduced traction of all four wheels induces excessive yaw during turning, and the vehicle's turning circle becomes unacceptably wide.
Additionally, in the case of a four-wheel drive vehicle which includes both a torque split controller and a traction controller, since a clutch is locked before effecting traction control to establish direct-coupled four-wheel driving, yaw control of the vehicle body by the torque split control is not effected during traction control, losing effect of provision of the torque split control. Further, since all of four wheels are connected directly to an engine to establish the four-wheel driving during the traction control, the four wheels slip simultaneously. Therefore, the traction control which selects the lowest speed from among the four wheels cannot operate effectively, resulting in control no different from a system utilizing an average wheel speed.
It will be appreciated that clutch control of a torque split controlled four-wheel drive vehicle is operable to allow a difference in rotational speeds between the front and rear wheels which corresponds to a difference in turning circles thereof to occur. The driving torque to the rear wheels is increased during turns with high lateral acceleration to induce positive slip of the rear wheels to control yaw motion of the vehicle body.
Therefore, traction control which reduces an engine power in response to slippage of the rear wheels in addition to the torque split control which positively promotes the driven wheel slippage causes disenabling of the essential yaw control effected by the torque split control. In other words, these two control systems work against each other, preventing effective utilization of both types of control.